Image processing apparatus and method which prevents the generation of a white stripe on an output image

ABSTRACT

An image processing apparatus for color-correcting image data for outputting to image forming means includes an input unit for inputting image data, a discrimination unit for determining whether or not the image data is of a predetermined color, a color correction unit for color-correcting the image data even if the image data is lower than a predetermined level in such a manner that a light emitting device in the image forming unit emits light at a small light emission level, and an output unit for outputting the color-corrected image data or image data generally representing white to the image forming means based on the result of the determination.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to image processing apparatus and methodfor conducting color correction.

2. Related Background Art

In a prior art color image forming apparatus, a process of transferringa record image formed on a photoconductor drum by charging, lightexposure and development is repeated for a plurality of colors to form acolor image.

FIG. 38 shows a construction of a prior art color image formingapparatus.

In FIG. 38, a roller charger 102, a cleaning device 103, a developingdevice 104 and a transfer drum 105 are arranged around a photoconductordrum 101.

The developing device 104 is formed in a rotatable cylinder shape andfour developing units 106a, 106b, 106c and 106d are provided therein andyellow, magenta, cyan and black toners are contained in the respectivedeveloping units 106a to 106d.

FIG. 39 shows a construction of the developing unit 106 (106a to 106d)and an application roller 107, a developing roller 108, a tonerrestriction member 109 and a toner container 110 are provided therein.As shown in FIG. 38, the developing roller 108 is exposed to theexternal from openings 111a to 111d of the developing units 106a to106d. The application roller 107 is rotated to apply the toner containedin the toner container 110 to the developing roller 108, and necessarytoribo is applied to the toner by the toner restriction member 109.

The transfer drum 105 comprises a resilient layer 113 on a metalcylinder 112 and a PVDF 114 on the resilient layer 113. A sheet feedroller 115, a gripper 116, a suction roller 117, a separation pawl 118,a fixing device 119, a cleaning device 120 and a discharging roller 121are arranged around the transfer drum 105.

An optical unit 122 and a reflection mirror 123 are arranged above thephotoconductor drum 101. The optical unit 122 comprises a laser diode, alaser driver, a rotating polygon mirror which is rotated at a high speedand a lens.

A sheet cassette 123 for containing transfer sheets, not shown, isarranged below the photoconductor drum 101.

The photoconductor drum 101 is driven in a direction a at a peripheralvelocity of 100 min/sec by drive means, not shown. The photoconductordrum 101 has a photoconductor of organic photoconductor (OPC) materialapplied on an outer periphery of an aluminum cylinder having a diameterof 40 mm. A-Si, CdS or Se may be used as the OPC.

The material of the toner restriction member 109 is nylon when the toneris charged negatively, and silicone rubber when the toner is chargedpositively such that a material which is charged in the oppositepolarity to that of the toner is used. A peripheral velocity of theapplication roller 107 is selected within a range of 1.0 to 2.0 times ofthe peripheral velocity of the photoconductor drum 101.

The transfer drum 105 has a resilient layer 113 of foamed urethanehaving a thickness of 2 mm wrapped on a metal cylinder 112 having adiameter of 156 mm and a PVDF 114 having a thickness of 100 μm iswrapped thereon.

A prior art color image forming apparatus is disclosed in Laid-openJapanese Patent Application No. 50-50935. The developing device 104 isdisclosed in Laid-open Japanese Patent Application No. 50-93437.

An operation in the above construction is now explained.

When the laser diode in the optical unit 122 is driven by a yellow imagesignal through the laser driver, the laser beam illuminates thephotoconductor drum 101 through the reflection mirror 123.

An AC voltage of 1500 V peak-to-peak at a frequency of 1000 Hz issuperimposed on a DC voltage of -700 V and the surface of thephotoconductor drum 101 is uniformly charged to approximately -700 V.The illuminated area of the photoconductor drum 101 is at approximately-100 V and an electrostatic latent image is formed. As thephotoconductor drum 101 is advance along the arrow a, the toner isdeposited to the electrostatic latent image by the developing device106a containing the yellow toner so that it is visualized.

On the other hand, a transfer sheet (not shown) fed from the sheetcassette 123 by the sheet feed roller 115 is held by a gripper 116 andthen electrostatically sucked to the transfer drum 105 by the suctionroller 117 to which the voltage is applied. The toner image on thephotoconductor drum 101 is transferred to the transfer sheet sucked tothe transfer drum 105 by the voltage applied to the transfer drum 105from a power supply, not shown.

The above process is repeated for the respective colors of magenta, cyanand black to form multi-color toner images superimposed on the transfersheet. The transfer sheet is scraped off the transfer drum 106 by theseparation pawl 110 and then fused and fixed by the fixing device byheating and pressurizing to form a full color image.

The remaining non-transferred toners on the photoconductor drum 101 arecleaned by the cleaning device 103 including a fur brush and blademeans. The photoconductor drum 101 is discharged by the dischargingdevice and initialized. In the present example, the charging roller 102is used for the charging of the photoconductor drum 101, and when thephotoconductor drum 101 is discharged, the DC voltage is set toapproximately 0 V while keeping the applied AC voltage as is.

The toners on the transfer drum 105 are also cleaned by the cleaningdevice 120 including the fur brush and the web. The transfer drum 105 isdischarged by the discharging roller 121 and initialized.

The developing method is preferably a one-component developing systemwhich does not require a complex construction such as an ATR or a screwand allows the use of a process cartridge system which enhances usermaintenance. Of the one-component development system, a non-contactdevelopment system offers an advantage of simple construction.

The color image forming apparatus described above is for a contactdevelopment system in which the developing roller 108 and thephotoconductor drum contact so that one of them must be resilient. Inthe non-contact developing system, however, those members may be analuminum substrate and hence a cost merit is high.

Further, since the color toner renders the tonality of the output imagebetter, it is desirable to use a toner of the sharp melt type which isinstantly molten at certain fixing temperature. However, this type oftoner often lowers a glass transition point and in the contactdeveloping system, toner may be fused to one or both of thephotoconductor drum 101 and the developing roller 108 by the abrasion ofthe photoconductor drum 101 and the developing roller 108. In order toprevent the fusing, it is desirable to use the non-contact developingsystem.

FIG. 40 shows a non-contact developing system in which four developingdevices 202a, 202b, 202c and 202d are fixedly arranged around thephotoconductor drum 101 and a color image may be formed without contactbetween the photoconductor drum 101 and the developing devices 202a to202d.

When a color image is formed by using the above non-contact developingsystem, the inventors of the present invention found that a whiteclearance which should not be present was created between colors of theimage formed by different adjacent colors and a white stripe was createdas shown in FIG. 41. This is caused because the visualized image isformed narrower than the electrostatic latent image formed on thephotoconductor drum when the latent image, for example, the image edgeat which a drum surface potential abruptly changes is formed on thephotoconductor drum and developed by the developing device. Inmonochromatic image formation, the narrowing of the image, even if itoccurs to some extent, does not cause a problem because there is noadjacent color.

However, when a color image is formed under such a condition and a cyanband and a black band, for example, are adjacent in the image, the imagein which the cyan band and the black band should appear adjacentlyincludes a clearance between the cyan band and the black band in thefinal image on the transfer sheet because the visualized cyan image andthe visualized black image are narrowed respectively.

As shown in FIG. 42, such narrowing of the image is a phenomenon causedby the narrowing of the edge as shown by the visualized area because theelectric field is wrapped at the edge (shown as the latent image area)of the electrostatic latent image formed on the photoconductor drum, andthe affect appears more prominently in the non-contact developingsystem.

The present invention is intended to solve the above problems and it isan object of the present invention to provide a color image formingapparatus which eliminates the narrowing of the image.

In order to achieve the above object, the present invention provides animage processing apparatus for color-correcting image data foroutputting to an image forming means, comprising input means forinputting image data; discrimination means for determining whether ornot the image data is of a predetermined color; color correction meansfor color-correcting the image data even if the image data is lower thana predetermined level in such a manner that a light emitting device inthe image forming means emits a light at a small light emission level;and output means for outputting the color-corrected image data or imagedata generally representing white to the image forming means based onthe result of the determination.

It is other object of the present invention to prevent the narrowing ofthe image without causing a fogging phenomenon in a non-print area.

In order to achieve the above object, the present invention provides animage processing apparatus comprising input means for inputting colorimage data; edge detection means for detecting an edge based on theinput color image data; and control means for controlling a lightemitting device in an image forming unit to emit light at a smallemission level for pixels near the edge.

Other objects and features of the present invention will be apparentfrom the following description of the embodiments and the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram of an Embodiment 1;

FIGS. 2A and 2B show waveforms of potentials on a photoconductor drum toillustrate a principle of the present invention;

FIG. 3 shows a block diagram of a system of the Embodiment 1;

FIG. 4 shows a block diagram of a configuration of a printer controller;

FIG. 5 shows a block diagram of a configuration of a printer engine;

FIG. 6 shows a correction table of a gamma correction unit;

FIG. 7 shows a timing chart of an operation of the circuit of FIG. 5;

FIG. 8 shows a block diagram of a configuration of an RF circuit of FIG.5;

FIG. 9 shows an address map of a ROM of FIG. 8;

FIGS. 10A to 10D show graphs to illustrate color conversion;

FIG. 11 shows a circuit diagram of a configuration of a white datadiscrimination circuit;

FIG. 12 shows a circuit diagram of a configuration of a selector;

FIG. 13 shows RGB image data;

FIG. 14 shows YMCK image data;

FIG. 15 shows a block diagram of an Embodiment 2;

FIG. 16 shows a block diagram of a configuration of a printer controllerof FIG. 15;

FIG. 17 shows a block diagram of a configuration of a printer engine;

FIG. 18 shows a configuration of a system of the Embodiment 2;

FIG. 19 shows a configuration of a controller;

FIG. 20 shows a block diagram of signal processing of the printerengine;

FIG. 21 shows a timing chart of signals of the printer engine;

FIG. 22 shows a LUT of a gamma correction unit;

FIG. 23 shows a configuration of a RF circuit;

FIG. 24 shows a configuration of a ROM of the RF circuit;

FIG. 25 shows a configuration of an edge detection unit;

FIG. 26 shows a circuit diagram of a register circuit;

FIG. 27 shows a circuit diagram of a pattern matching circuit;

FIG. 28 shows a circuit diagram of an image adding circuit;

FIG. 29 illustrates a pattern matching process;

FIG. 30 shows image data before conversion;

FIG. 31 shows image data after conversion;

FIGS. 32A and 32B illustrate an effect of the Embodiment 2;

FIG. 33 shows a configuration of a system of the Embodiment 2;

FIG. 34 shows a block diagram of signal processing of a printer engineof a Modification 1;

FIG. 35 shows a configuration of a system of the Modification 1;

FIG. 36 shows a configuration of a controller of the Modification 1;

FIG. 37 shows a block diagram of signal processing of a printer engineof a modification of the Embodiment 2;

FIG. 38 shows a configuration of a color image forming apparatus by aprior art contact developing system;

FIG. 39 shows a configuration of a developing device of FIG. 38;

FIG. 40 shows a configuration of a color image forming apparatus by aprior art non-contact developing system;

FIG. 41 shows a color image formed by the color image forming apparatusby the prior art non-contact developing system; and

FIG. 42 shows a manner of an electric field at a portion of the colorimage forming apparatus by the prior art non-contact developing system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[Embodiment 1]

FIG. 1 shows a block diagram of an image processing apparatus of anEmbodiment 1.

In FIG. 1, numeral 201 denotes a laser light source for generating alaser beam, numeral 202 denotes a photoconductor drum on which anelectronic latent image is formed by directing the laser beam from thelaser light source, numeral 203 denotes developing means for depositingtoner on the photoconductor drum to develop the latent image to form atoner image, and numeral 204 denotes transfer means for transferring thetoner image to a transfer sheet.

Numeral 205 denotes color conversion means for converting RGB image datato YMCK (yellow, magenta, cyan and black) image signal and numeral 206denotes laser control means for producing a laser control signal forcontrolling the light emission of the laser light source 201 inaccordance with the YMCK image signal.

An operation is now explained.

The input RGB image signal is converted to the YMCK image signal by thecolor conversion means 205. The laser control means 206 controls thelight emission of the laser light source 201 in accordance with the YMCKimage signal.

The photoconductor drum 201 is illuminated by the laser beam to form anelectronic latent image thereon and the developing means 203 depositstoners on the photoconductor drum 201 to form a toner image. The tonerimage is transferred to the transfer sheet by the transfer means 204.

The above operation is conducted for each color of YMCK so that a colorimage is formed on the transfer sheet.

A YMCK image signal which relieves an electric field wrapping phenomenonwhich causes an edge narrowing phenomenon is created by the colorconversion means 205 in accordance with the input RGB image signal.

A concept of the method for relieving the electric field wrappingphenomenon is described below.

FIGS. 2A and 2B show surface potentials on the photoconductor drum 202.

FIG. 2A shows a surface potential in prior art image formation. Apotential in a print area is set to approximately -100 V and a potentialin a non-print area is set to -700 V. FIG. 2B shows a surface potentialin the image formation by the present invention. A potential in theprint area is set to -100 V and a potential in the non-print area is setto -700 V. In the condition of FIG. 2A, the potential abruptly changesat a boundary (point A) of the print area and the non-print area asdescribed above so that strong wrapping of the electric field occurs. Bydirecting fine laser emission to the non-print area as shown in FIG. 2B,the change of the potential at the boundary of the print area and thenon-print area is stepwise and the wrapping of the electric field may beweakened. Accordingly, since the narrowing of the visualized image onthe photoconductor drum in the non-contact developing system isprevented, the generation of a white stripe by the clearance createdbetween different color areas is prevented.

When the laser light source 201 is operated in a fine light emissionmode, it is effected within a range in which the surface potential ofthe photoconductor drum by the fine light emission does not affect theimage quality of the color image.

FIG. 3 shows a block diagram of the Embodiment 1 of the presentinvention based on the above principle.

In FIG. 3, image information in a predetermined language derived from ahost computer 1 as an external unit is received by a printer 2 as acolor image forming apparatus. The printer 2 comprises a printercontroller 3 and a printer engine 4.

The image information is received by the printer controller 3 whichdevelops the image and sends the image data 5 to the printer engine 4.The printer engine 4 prints the data based on the image data 5 and formsa full color image on the transfer sheet.

In the following description, it is assumed that the image data 5 ismulti-value image data representing 8-bit R (red), G (green) and B(blue) brilliance, respectively.

Major signals sent from the printer controller 3 to the printer engine 4are the image data 5 (RDATA0 to RDATA7, GDATA0 to GDATA7 and BDATA0 toBDATA7), an image transfer clock (VCLK), a line sync signal (LSYNC) anda page sync signal (PSYNC). It is assumed that the printer engine 4prints at a resolution of 600 dpi (dots per inch).

FIG. 4 shows a configuration of the printer controller 3.

The image signal sent from the host computer 1 is developed by an imagedeveloping unit 6 into R-G-B multi-value brilliance data and one page ofdata is temporarily stored in an image memory 7 and then the image datais read and transferred to the printer engine 4.

FIG. 5 shows a configuration of a signal processing unit of the printerengine 4.

The R, G, B image data sent from the printer engine 4 is color-convertedto magenta (M), cyan (C), yellow (Y) and black (K) by an RF(reproduction function) circuit 8 and is outputted as field sequentialimage data 9 in the order of M, C, Y and K. The image data 9 is writteninto a line memory 10 by an image transfer clock VCLK and then read by aclock PCLK and sent to a gamma correction unit 11. The clock PCLK isgenerated by a control clock generation unit 12.

The gamma correction unit 11 comprises a gamma correction table as alook-up table (LUT) formed by a RAM or a ROM and the read image data 9is inputted to addresses A0 to A7 and a color designation signal 13 isinputted to A8 and A9. An address map of the gamma correction table isshown in FIG. 6. As shown in FIG. 6, the gamma correction conductsdifferent corrections depending on the colors of the toners.

Thus, the gamma correction table is set in accordance with a processconducted in the RF circuit 8 to be described later.

The gamma-corrected image data as shown in FIG. 7 is converted to ananalog signal 16 by a D/A converter 15 and is inputted to a positiveinput of a comparator 17. A ramp signal 19 shown in FIG. 7 which isgenerated by a ramp wave generator 18 in synchronism with the clock PCLKis inputted to a negative input of the comparator 17.

The comparator 17 compares the image signal 16 with the ramp signal 19to output a laser control signal 20 which is PWM-modulated as shown inFIG. 7. The laser control signal 20 controls the light emission of thelaser diode through the laser driver so that the potential on thesurface of the photoconductor drum changes with the pulse width of thelaser control signal 20 as shown in FIG. 7.

The printing is conducted in areas in which the surface potential of thephotoconductor drum exceeds a predetermined print threshold. In theexample of FIG. 7, when the image data 14 is 00_(H) and 01_(H), thepulse width of the laser control signal 20 is narrow and theillumination time of the laser beam to the photoconductor drum is shortso that the surface potential does not exceed the print threshold andthe printing is not conducted. Namely, the toner image is not formed onthe transfer sheet. When the image data 14 is 80_(H) and FF_(H), thepulse width is wide and the illumination time of the laser beam is longso that the surface potential exceeds the print threshold and the tonerimage is formed on the transfer sheet.

FIG. 8 shows a configuration of the RF circuit 8.

In FIG. 8, numeral 21 denotes a white data discrimination circuit fordetermining whether or not the input RGB image data 5 is white toproduce a white data discrimination signal 22, numerals 24, 25 and 26denote ROMs having logarithmic conversion LUTs, numerals 27, 28, 29 and34 denote switches controlled by a mode selection signal (MODE) 23,numeral 30 denotes a UCR (under color removal) circuit, numeral 31denotes a masking circuit including a product sum circuit and numeral 32denotes a ROM having a LUT of masking coefficients and UCR coefficients.An address map thereof is shown in FIG. 9. Numeral 33 denotes a selectorand numeral 35 denotes a control signal.

FIG. 11 shows a configuration of the white data discrimination circuit21.

In this circuit, 8-bit B, G and R bits are inputted to multi-input ANDgates 40, 41 and 42, respectively, to detect B=FF_(H), G=FF_(H) andR=FF_(H), respectively, and the respective AND outputs are inputted toan AND gate 43 to output the white data discrimination signal 22 whichassumes a H (high) level for the white data.

FIG. 12 shows a configuration of the selector 33.

This circuit comprises AND gates 44 to 47 and 49 and an OR gate 48 asshown. When the white data discrimination circuit 21 discriminates thewhite data, the YMCK data is outputted as 00_(H) without regard to theYMCK image signal inputted to the selector 23.

On the other hand, when the white data discrimination circuit 21discriminates color data, the YMCK image signal inputted to the selector33 corresponding to the color designation signal is outputted.

An operation when the white data discrimination circuit 21 discriminatesdata other than white data is now specifically explained. It is assumedthat the switches 27, 28, 29 and 34 are closed to the position a of thecolor mode by the MODE signal 23. The 8-bit R, G and B image data 5outputted by the printer controller is logarithmically controlled by theLUTs stored in the ROMs 24, 25 and 26 so that blue (B) isdensity-converted to yellow (Y), green (G) to magenta (M) and red (R) tocyan (C), respectively, and are inputted to the UCR circuit 30 throughthe switches 27, 28 and 29, respectively.

When a control signal 35 of `100` is inputted, it is sent to A8 to A10of the ROM 32 shown in FIG. 9 to select the magenta (M) UCR table, andthe UCR circuit 30 detects a minimum value of the input Y, M and C 8-bitdata. The detected minimum value is sent to A0 to A7 of the ROM 32 toaddress it and the magenta (M) UCR data corresponding to the input datais outputted from DATA.

When the control signal 35 of `000` is inputted, it is sent to A8 to A10of the ROM 32 to set a bank and the address data is sent from theregister of the masking circuit 31 to A0 to A7 of the ROM 32 so that theROM 32 sets the addressed magenta (M) masking coefficient data to themasking circuit 31. The magenta (M) image data outputted from the UCRcircuit 30 is multiplied and summed with the masking coefficient set bythe masking circuit 31 and the result is outputted to the selector 33.Then, the selector 33 is switched by the color designation signal 13 andthe magenta (M) image data 9 is outputted to a succeeding stage throughthe switch 34.

When the above operation is completed for one field, the control signal35 of `101` is then sent to A8 to A10 of the ROM 32 to select the cyan(C) UCR table, and a minimum value of Y, M and C is detected by the UCRcircuit 30 and is sent to A0 to A7 of the ROM 32 to address it, and thecyan (C) UCR data corresponding to the input data is outputted fromDATA. Then, the control signal 35 of `000` is sent to A8 to A10 of theROM 32, the address data is sent from the register of the maskingcircuit 31 to A0 to A7 of the ROM 32 and the ROM 32 sets the addressedcyan (C) masking coefficient data to the masking circuit 31. The cyan(C) image data outputted from the UCR circuit 30 is multiplied andsummed with the masking coefficient set by the masking circuit 31 andthe result is outputted to the selector 33. The selector 33 is thenswitched by the color designation signal 13 and the cyan (C) image datais outputted to the succeeding stage. This operation is conducted forone field of data.

Then, the control signal 35 of `110` is sent to A8 to A10 of the ROM 32to select the yellow (Y) UCR table, the UCR circuit 30 detects a minimumvalue of Y, M and C and sends the minimum value to A0 to A7 of the ROM32 to address it, and the yellow (Y) UCR data corresponding to the inputdata is outputted from DATA. Then, the control signal 35 of `000` issent to A8 to A10 of the ROM 32 to set the bank, the address data issent to A0 to A7 of the ROM 32 from the register of the masking circuit31 and the ROM 32 sets the addressed yellow (Y) masking coefficient datato the masking circuit 31. The yellow (Y) image data outputted from theUCR circuit 30 is multiplied and summed with the masking coefficient setby the masking circuit 31 and the result is outputted to the selector33. The selector 33 is switched by the color designation signal 13 andthe yellow (Y) image data is outputted to the succeeding stage. Thisoperation is conducted for one field of data.

Then, the control signal 35 of `111` is sent to A8 to A10 of the ROM 32to select the black (K) UCR table, the UCR circuit 30 detects a minimumvalue of Y, M and C and sends the minimum value to A0 to A7 of the ROM32 to address it and the black (K) UCR data corresponding to the inputdata is outputted from DATA. Then, the black (K) image data outputtedfrom the UCR circuit 30 is outputted to the selector 33. The selector 33is switched by the color designation signal 13 and the black (K) imagedata is outputted to the succeeding stage. This operation is conductedfor one field of data.

By the four-step operation described above, one field of colorconversion process is completed.

The UCR and masking coefficients for the Y, M, C and K colors used inthe color mode are preset such that the Y, M, C and K image signalsafter color correction are larger than a predetermined value, 10_(H) inthe illustrated example, without regard to the input RGB image signal.

When the image data is monochromatic, the switches 27, 28, 29 and 34 areconnected to a position b by the MODE signal 23 so that the R, G and Bimage data is inputted to the UCR circuit 30 and directly inputted tothe masking circuit 31. Then, the control signal 35 of `000` is sent toA8 to A10 of the ROM 32 to set the bank, the address data is sent fromthe register of the masking circuit 31 to A0 to A7 of the ROM 32 and theROM 32 sets the addressed brilliance conversion coefficient data to themasking circuit 31. The image data is brilliance converted in the samemanner as that for the color image and it is outputted from the selector33. Then, the control signal 35 of `010` is sent to A8 to A10 of the ROM32 to set the bank to the black-white mode, the data outputted from theselector 33 is sent to A0 to A7 of the ROM 32 to address it, and thelogarithmically converted data corresponding to the input data isoutputted from DATA. By this operation, the image in the black-whitemode is outputted.

The logarithmic conversion data used in the monochromatic mode has beenconverted in accordance with the brilliance data level inputted to theROM 32 unlike the data outputted from the ROMs 24, 25 and 26 in thecolor mode.

Referring to FIGS. 10A to 10D, a specific example of a process result bythe RF circuit is explained.

FIGS. 10A to 10D show color conversion results to the YMCK data when Gdata=FF_(H), B data=FF_(H) and R data is 00_(H) to FF_(H).

In FIG. 10A, when R=FF_(H), it is discriminated as the white data andall YMCK data are set to 00_(H). When R=FF_(H), it is discriminated asthe non-white data and all YMCK data are set to 10_(H). Accordingly, thephotoconductor drum is illuminated with a pulse width of the lasercontrol signal 20 corresponding to 10_(H). As explained with referenceto FIG. 7, since the pulse width for 10_(H) is narrow, the surfacepotential of the photoconductor drum is not changed to the extent thatthe toner is deposited. As a result, the surface potential of thephotoconductor drum is slightly changed without affecting the tonalityof the printed image so that the wrapping of the electric fielddescribed above is reduced and the narrowing of the visualized image onthe photoconductor drum is prevented. Similarly in FIGS. 10B to 10D,when C, Y, K is FF_(H) is discriminated as the white data and when C, Y,K is FE_(H), it is discriminated as the non-white data and the lasercontrol signal 20 by 10_(H) is outputted to prevent the narrowing of thevisualized image.

FIG. 13 shows RGB image data and FIG. 14 shows YMCK image data convertedfrom the RGB image data of FIG. 13. As shown in FIG. 14, when all of RGBare FF_(H), it is discriminated as the white data and all of YMCK areset to 00_(H). When not all of RGB are FF_(H), the RGB-YMCK conversionis conducted such that YMCK have high densities when they are above10_(H) so that the laser weakly emits light for all of YMCK.

In accordance with the present embodiment, the white stripe due to theclearance created between different color areas as shown in FIG. 21which is caused by the narrowing of the edge is prevented and a highgrade image is provided.

By providing the white data discrimination circuit 21 to differentlyprocess the white data and the non-white data, the processes appropriatefor the respective data may be conducted. Namely, for the white data,even if the edge is narrowed, the white image area somewhat increasesand it does not cause a significant problem. Rather, the enhancement ofthe edge which is the boundary of the white image area and the colorimage area may provide a high grade image.

[Modification]

In the above embodiment, the white data and the color data arediscriminated and the process is changed depending on the discriminationresult. The present invention is not limited thereto but the white dataneed not be discriminated and the above process may be uniformlyconducted for the entire image data.

The circuit may be configured as shown in FIG. 15.

In FIG. 15, the RF circuit 8 in the above embodiment is built in theprinter controller 50. In the printer controller 50, the RGB brilliancesignals are converted to the YMCK density signals and the YMCK imagedata 9 is sent to the printer engine 51.

FIG. 16 shows a configuration of the printer controller 50.

In FIG. 16, the RF circuit receives the RDATA (7:0), the GDATA (7:0) andthe BDATA (7:0) of the image data 5 sent from the image memory 7 andoutputs the YMCK image data 9.

The succeeding stage printer engine 51 receives the YMCK image data 9and forms a color image in accordance with the received signal. FIG. 17shows a configuration of the printer engine 51 in which the RF circuit 8in FIG. 5 is omitted. The configuration of the RF circuit of FIG. 16 andthe processing method are identical to those of the Embodiment 1.

An advantage of the present embodiment resides in that the number ofimage signal lines of the interface between the printer controller 50and the printer engine 51 is eight compared with 24 in the Embodiment 1.

In accordance with the present embodiment, a narrowing of the image isprevented and the high grade image is outputted.

Further, the white stripe is prevented by the appropriate colorcorrection based on the input image data and the edge at the boundary ofthe white image area and the color image area is kept so that the highgrade image is outputted.

[Embodiment 2]

In the Embodiment 2, in order to relieve the electric field wrappingphenomenon, the charging potential to uniformly charge the drum surfaceis lowered. In accordance with the Embodiment 1, the narrowing of thevisualized image may be reduced but the deposition of the toner to thenon-print area or a so-called fogging phenomenon occurs and a sufficientimage density may not be attained because the potential differencebetween the print area and the non-print area is small and thoseproblems are to be resolved.

In the Embodiment 2, it is intended to prevent the narrowing of theimage without causing the fogging phenomenon in the non-print area.

FIG. 18 shows a block diagram of the Embodiment 2. A printer 1002receives image information in a predetermined language from a hostcomputer 1001 as an external unit and a printer controller 1003 developsthe image and sends the image data 1007 to a printer engine 1004. Theprinter engine 1004 print the data in accordance with the image data1007 to form a full color image. In the following description, it isassumed that the image data 1007 sends three-color data, red (R), green(G) and blue (B) and the printer engine 1004 prints the image at aresolution of 600 dpi (dots per inch).

In FIG. 18, major signals exchanged between the printer controller 1003and the printer engine 1004 are color brilliance signals 7 (RDATA0 toRDATA7, GDATA0 to GDATA7 and BDATA0 to BDATA7), a number of linesdesignation signal 1042, an image transfer clock (VCLK) and a page syncsignal (PSYNC).

FIG. 19 shows a block diagram of the printer controller 1003. The imagedeveloping unit 1005 develops the image data in the predeterminedlanguage sent from the host computer 1001 into R, G and B multi-valuebrilliance data and generates the number of lines designation signal foreach pixel. One page of data is stored in a multi-value image memory1006 and 24-bit brilliance data, 8 bits for each of R, G and B is sentto the printer engine 1004.

FIG. 20 shows a block diagram of a signal processing unit of the printerengine 1004. The multi-value image data 1007 sent from the printercontroller 1003 described above is color-converted by an RF(reproduction function) circuit 8 to magenta (M), cyan (C), yellow (Y)and black (K) image data and image data 1008 is outputted in the orderof M, C, Y and K and is written into a line memory 1009 and read insynchronism with a rise of the image clock (PCLK) of the printer engine.The outputted multi-value image data 1017 is outputted to a gammacorrection unit 1010 through an edge detection unit. The gamma detectionunit 1010 comprises a look-up table (LUT) formed by a RAM or a ROM andthe image data is inputted to addresses A0 to A7, the number of linesdesignation signal is inputted to A10 and the color designation signalis inputted to A8 and A9. An address map of the gamma correction tableis shown in FIG. 22.

The gamma correction unit 1010 conducts the gamma correction inaccordance with the color components of the 8-bit multi-value imagesignal 1041 designated by the color designation signal 1018 and thenumber of lines designated by the number of lines designation signal1042. The 8-bit multi-value image signal 1053 from the gamma correctionunit 1010 is converted to an analog voltage by a D/A conversion unit1013 and is inputted to positive inputs of succeeding stage comparators1014 and 1047. An output signal from a 600-line ramp signal generator1012 is inputted to a negative input of the comparator 1014. The600-line ramp signal generator 1012 converts the image clock PCLK to aramp wave by an integration circuit. The comparator 1014 outputs a600-line center growing PWM signal 1051 which is inputted to oneterminal of a selector 1048.

On the other hand, an output from a 200-line ramp signal generator 1046is inputted to a negative input of the comparator 1047. The 200-lineramp wave generator 1046 divides the image clock PCLK by three andconverts it to a ramp signal by an integration circuit. The comparator1047 outputs a 200-line center growing PWM signal 1052 which is inputtedto the selector 1048. The selector 1048 selects one of the 200-line PWMsignal and the 600-line PWM signal based on the number of linesdesignation signal 45.

A timing chart of the signals in the circuit diagram of FIG. 20 is shownin FIG. 21. As shown, which one of the 200-line PWM signal and the600-line PWM signal is selected as the laser drive signal is determinedby the number of lines designation signal 1045.

FIG. 23 shows a block diagram of the RF circuit 1008. Numerals 1024,1025 and 1026 denote ROMs having logarithmic conversion LUTs, numerals1027, 1028, 1029 and 1034 denote switches controlled by the modeselection signal (MODE), numeral 1030 denotes a UCR (under colorremoval) circuit, numeral 1031 denotes a masking circuit including aproduct-sum circuit, numeral 1032 denotes a ROM having a LUT for maskingcoefficient and UCR efficient (an address map thereof is shown in FIG.24) and numeral 1033 denotes a selector.

Detail of the operation of the RF circuit is now explained. The R, G, B8-bit multi-value brilliance data 1007 outputted from the printercontroller are logarithmically converted by the LUTs stored in the ROMs1024, 1025 and 1026 such that blue (B) is density-converted to yellow(Y), green (G) to magenta (M) and red (R) to cyan (C), and are inputtedto the UCR circuit 1030.

The table of the UCR circuit 1030 and the masking coefficients of themasking circuit 1031 are set appropriately from the tables and maskingcoefficients stored in the ROM 1032 based on the control signal.

Address data is sent to A0 to A7 and the cyan (C) UCR data correspondingto the input data is outputted from DATA. Then, the control signal issent to A8 to A10 of the ROM 1032 to set the bank, the address data issent from the register of the masking circuit 1031 to A0 to A7 of theROM 1032 and the ROM 1032 sets the masking coefficient of the addressedcyan (C) to the masking circuit 1031. The cyan (C) image data outputtedfrom the UCR circuit 1030 is multiplied and summed with the maskingcoefficient set by the masking circuit 1031 and the result is outputtedto the selector 1033. The selector is then switched by the colordesignation signal and the cyan (C) image data is outputted to thesucceeding stage. This operation is conducted for one field of data.Then, the control signal is sent to A8 to A10 of the ROM 1032 to selectthe yellow (Y) UCR table, the UCR circuit 1030 detects a minimum valueof Y, M and C and sends the minimum value to A0 to A7 of the ROM 1032 toaddress it and the yellow (Y) UCR data corresponding to the input datais outputted from DATA. Then, the control signal is sent to A1 to A10 ofthe ROM 1032 to set the bank, the address data is sent from the registerof the masking circuit 1031 to A0 to A7 of the ROM 32 and the ROM 32sets the addressed yellow (Y) masking coefficient data to the maskingcircuit 1031. The yellow (Y) image data outputted from the UCR circuit1030 is multiplied and summed with the masking coefficient set by themasking circuit 1031 and the result is outputted to the selector 1033.The selector is switched by the color designation circuit and the yellow(Y) image data is outputted to the succeeding stage. This operation isconducted for one field of data. Then, the control signal is sent to A8to A10 of the ROM 1032 to select the black (K) UCR table, the UCRcircuit 1030 detects a minimum value of Y, M and C and sends the minimumvalue to A0 to A7 of the ROM 1032 to address it and the black (K) UCRdata corresponding to the input data is outputted from DATA. Then, theblack (K) image data outputted from the UCR circuit 1030 is outputted tothe selector 1033. Then, the selector 1033 is switched by the colordesignation signal and the black (K) image data is outputted to thesucceeding stage. This operation is conducted for one field of data. Bythe four-step operation described above, one field of color conversionprocess is completed.

Specifically, the control signal is sent to A8 to A10 of the ROM 1032 toselect the magenta (M) UCR table and the UCR circuit 1030 detects aminimum value of the input Y, M and C 8-bit data. The detected minimumvalue is sent to A0 to A7 of the ROM 1032 and the magenta (M) UCR datacorresponding to the input data is outputted from DATA. Then, thecontrol signal is sent to A8 to A10 of the ROM 32 to set the bank, theaddress data is sent from the register of the masking circuit 1031 to A0to A7 of the ROM 1032 and the ROM 1032 sets the addressed magenta (M)masking coefficient data to the masking circuit 1031. Then, the magenta(M) image data outputted from the UCR circuit 1030 is multiplied andsummed with the masking coefficient set by the masking circuit 1031 andthe result is outputted to the selector 1033. Then, the selector isswitched by the color designation signal and the magenta (M) image datais outputted to the succeeding stage. This operation is conducted forone field of data and then the control signal is sent to A8 to A10 ofthe ROM 1032 to select the cyan (C) UCR table, the UCR circuit 1030detects a minimum value of Y, M and C and sends the minimum value to A0to A7 of the ROM 1032 to address it. When the image data ismonochromatic, the switches 1027, 1028, 1029 and 1034 are connected tothe position B by the MODE signal so that the R, G and B multi-valueimage data is inputted to the UCR circuit 1030 and directly inputted tothe masking circuit 1031. Then, the control signal is sent to A8 to A10of the ROM 1032 to set the bank, the address data is sent from theregister of the masking circuit 1031 to A0 to A7 of the ROM 1032 and theROM 1032 sets the addressed brilliance converted coefficient data to themasking circuit 1031. The image data is brilliance-converted in the samemanner as that for the color image and outputted from the selector 1033.Then, the control signal is sent to A8 to A10 of the ROM 1032 to set theblack and white mode, the data outputted from the selector 1033 is sentto A0 to A7 of the ROM 1032 to address it and the logarithmicallyconverted data corresponding to the input data is outputted from DATA.By this operation, the black and white mode image is outputted.

FIG. 25 shows a block diagram of the edge detection unit 1040. The edgedetection unit comprises three units, a register circuit 1054, an imageadding circuit 1055 and a pattern matching circuit 1056. FIG. 26 shows acircuit diagram of the register circuit 1054. It has a function to hold8 bits, in a main scan direction, of sequentially sent 8-bit imagesignal 1017 and a number of lines designation signal 44. In thiscircuit, the image signal is delayed by four dots and sent to thesucceeding stage image adding circuit 1055. Of the image signaloutputted from 9-bit flip-flops 58 to 65, the most significant bit (VDO(7)) is sent to the pattern matching circuit 1056.

FIG. 27 shows a circuit diagram of the pattern matching circuit. When animage condition shown in FIG. 29 is met, an edge detection signaloutputted from an OR circuit 1072 is High. Signals a to i are Low whenthe multi-value image signal is 00(H) to 7F(H) and High when themulti-value image signal is 80(H) to FF(H).

FIG. 28 shows the image adding circuit. In FIG. 28, numeral 1075 denotesan adder for adding 8-bit data and 8-bit data (which is fixed to 05(H)in the present circuit). When a sum exceeds FF(H), it is rendered toFF(H).

In this circuit, when the edge detection signal 57 is Low, the imagesignals VDO(7)-VDO(0) outputted from the register circuit 1054 and thenumber of line designation signal 1066 are outputted as is. When theedge detection signal 67 is High, 05(H) is added to the image signalsVDO(7)-VDO(0) sent from the register circuit 1054 and a sum image signal1041 and the Low number of lines designation signal 1045 (designatingthe 600-line PWM) are outputted. The image signal 1041 and the number oflines designation signal 1045 are inputted to the gamma correction unit1010. In the present circuit, the add value is set to 05(H) althoughthis value may be determined based on the gamma characteristic of theprinter.

FIG. 29 represents a matching condition of the pattern matching circuit.For a pixel e under consideration, the surrounding pixels along the mainscan direction are referred to determine the matching. Taking a pattern1 as an example, when the pixel e under consideration is a low densitypixel, pixels f to i are low density pixels and pixels a to d are highdensity pixels, the pixel e under consideration is determined as thematching pixel and a predetermined value (05(H) in the present example)is added to the original image data.

FIGS. 30 and 31 show manners of image data conversion in the presentembodiment. In FIG. 30, the signals (image data 1017 and number of linesdesignation signal 1044) are color-converted by the RF circuit 1008 andbuffered in the line memory 1009. M represents a magenta data value, Crepresents a cyan data value, Y represents a yellow data value, Krepresents a black data value and IMCHR represents a number of linesdesignation signal (which designates the 600-line by Low and the200-line by High). FIG. 31 shows the converted signals (image data 1041and the number of lines designation signal 1045) in the presentembodiment.

The process of conversion is now explained taking the third line of FIG.31 as an example. In printing the magenta color image data, the F dotthird line pixel (hereinafter referred to as F3) and the pixel I3 areconverted to 00(H) and 05(H). The pixel F3 matches to the pattern ofFIG. 12 and the pixel I3 matches to the pattern 3. The printing of theyellow color image data is conducted in the same manner as that for themagenta data. In the printing of the black color data, the pixels E3 andJ3 are converted from 00(H) to 05(H). The pixel E3 matches to thepattern 3 and the pixel J3 matches to the pattern 1. In any case, forthe pixel converted while the image data matches to the pattern 1 to 4,the number of lines designation signal is converted from High to Low.

A fine image less than 2 dots is not converted.

FIG. 32A shows a manner of depositing the toner on the sheet in theprior art. FIG. 32B shows a manner of depositing the toner on the sheetin the present embodiment. In both cases, the dots A to J of the thirdline are considered. As shown, in FIG. 32A, an area in which the toneris not deposited is present in the boundary of the image (point w) and awhite stripe appears but in FIG. 32B, the white stripe disappears and ahigh grade image is attained.

Namely, in accordance with the present embodiment, the pattern matchingalong the main scan direction is used for each color to detect the edgefor each color and the image level is raised to apply weak laseremission to the pixels near the edge to suppress the narrowing of theimage along the main scan direction due to the electric field wrappingwhich is the cause of the white stripe.

Further, since the edge detection is conducted based on the Y, M, C andK image data corresponding to the recording agents used in the printer,the edge at which the white stripe appears can be precisely detected.

Further, since the recording is made at a high resolution for the edgedetected by the edge detection unit 1040 without regard to the number oflines designation signal, the edge area is reserved and the high gradeimage is attained.

[Modification 1]

FIG. 33 shows a block diagram of a Modification 1 of the Embodiment 2.In the Modification the number of lines of the PWM in the Embodiment 2is only set to 200 lines. Accordingly, the number of lines designationsignal between the printer controller 1302 and the printer engine 1304in FIG. 33 is omitted.

FIG. 34 shows a block diagram of the printer engine 1304. As shown, onlyone circuit for conducting the PWM (the D/A converter 1309, thecomparator 1312 and the ramp wave generator 1311) for the 200 lines isprovided.

In this circuit configuration, the value to be added to the pixeldetermined as the edge of the image by the edge detection unit isrendered small so that the white stripe at the boundary of the colorimages is eliminated and the high grade image is attained.

As shown in FIGS. 35, 34 and 37, the printer controller 1402 may convertto the YMCK image and detect the edge and the color correction may bemade such that the laser emission is rendered small at the edge.

[Modification 2]

In the printer controller 3 of the Embodiment 2, the image signal in thepredetermined language from the host computer is developed in themulti-value image memory into the R, G and B brilliance image data andan outline is detected based on the R, G and B brilliance image data togenerate the number of lines designation signal to designate the200-line or the 600-line.

In accordance with the Modification 2, since the detection is made basedon the R, G and B brilliance image data which represents the originalimage with high fidelity, the high precision detection is attained.

On the other hand, since the edge detection to detect the edge at whichthe laser emission is rendered small is conducted based on the Y, M, Cand K image data, the area at which the white stripe appears can beprecisely detected.

Accordingly, in accordance with the Modification 2, for the image shownin FIG. 39, the image is formed with the 600-line for the edge along thesub-scan direction in FIG. 41.

In this manner, the white stripe is prevented and the outline can beformed with high precision.

[Modification 3]

The edge detection unit of the Embodiment 2 is provided with the 8-dotregister circuit and detects both of the falling end pixel and therising end pixel of the data level for each color component as shown inFIG. 27.

On the other hand, only the falling end pixel of the data level may bedetected for each color component and the predetermined value may beadded to the detected pixel under consideration as it is in theEmbodiment 2.

In the above Embodiment 2 and the Modification thereof, the laseremission is rendered small by the image processing near the edge.Alternatively, a control signal to render the laser emission small maybe directly applied to the laser driver to render the laser emissionsmall near the edge.

Further, since the narrowing is not very prominent at the boundary tothe white image area, the laser emission at the small level may beomitted for the boundary to the white image area so that the edge at theboundary of the white image area and the color image area may beenhanced.

In accordance with the Embodiment 2, the edge is detected and thecontrol is made to render the laser emission small near the edge so thatthe fogging phenomenon in the non-print area does not occur and thenarrowing of the image, that is, the white stripe is prevented.

Further, the fogging phenomenon in the non-print area is prevented, thenarrowing of the image, that is, the white stripe is prevented and theoutline is reserved.

The present invention is not limited to the above embodiments andvarious modifications thereof may be made without departing from thescope of the claims.

What is claimed is:
 1. An image processing apparatus comprising:inputmeans for inputting color image data; edge detection means for detectingan edge based on the input color image data; and control means forcontrolling a light emitting device in an image forming unit to emitlight at a small emission level for pixels near the edge, in order tonot form an image at the pixels near the edge.
 2. An image processingapparatus according to claim 1, wherein an image is not formed at thesmall light emission level of the light emission device in said imageforming unit.
 3. An image processing apparatus according to claim 1,wherein the color image data comprises color components corresponding torecording agents used in said image forming unit, and said edgedetection means detects the edge independently for each color component.4. An image processing apparatus according to claim 1, wherein saidimage forming unit is capable of forming the image with a plurality ofresolutions and forms the image at a high resolution for the edge.
 5. Animage processing apparatus according to claim 1, wherein said edgedetection means detects the edge along a main scan direction in theformation of the image.
 6. An image processing apparatus according toclaim 1, further comprising image forming means for forming the image byusing pulse width modulation and electronic photography.
 7. An imageprocessing apparatus comprising:input means for inputting color imagedata; outline detection means for detecting an outline of an image basedon the color image data; color conversion means for generating colordensity image data corresponding to recording agents used in an imageforming unit based on the color image data; edge detection means fordetecting an edge based on the color density image data; and controlmeans for controlling a light emitting device in said image forming unitto emit a light at a small light emission level for pixels near the edgein order to not form an image at the pixels near the edge, andcontrolling to form the image at a high resolution for the outline. 8.An image processing apparatus according to claim 7, wherein the colorimage data comprises color components corresponding to the recordingagents used in said image forming unit, and said edge detection meansdetects the edge independently for each color component.
 9. An imageprocessing apparatus according to claim 7, wherein said edge detectionmeans detects the edge along a main scan direction in the formation ofthe image.
 10. An image processing method comprising the stepsof:inputting color image data; detecting an edge based on the inputcolor image data; and controlling a light emitting device in an imageforming unit to emit a light at a small light emission level for pixelsnear the edge, in order to not form an image at the pixels near theedge.
 11. An image processing method comprising the steps of:inputtingcolor image data; detecting an outline of an image based on the colorimage data; generating a color density image data corresponding torecording agents used in an image forming unit based on the color imagedata; detecting an edge based on the color density image data; andcontrolling a light emitting device in the image forming unit to emit alight at a small light emission level for pixels near the edge in orderto not form an image at the pixels near the edge, and to form the imageat a high resolution for the outline.